Straddle cartridges for known straddle cartridge anchors are made of plastic in one piece. The straddle cartridges with the straddle members have to possess two diametrically opposed characteristics, namely, maximum hardness with a minimum of deformation of the straddle cartridge body so as to enable it to absorb the high pressure forces, and extreme flexibility coupled with a high degree of stiffness which prevent a sliding back of the anchor head due to the spring force and support at the bore wall thus assuring a safe known plastic straddle cartridges constitute a compromise between the opposite requirements. If the straddle cartridge was made of a hard plastic material the straddle members broke off and the anchor was not functional. If the straddle cartridge was made of a flexible plastic material, the pressure resistance did not meet the requirements.